Optical fiber devices, such as optical fiber lasers and amplifiers, that can provide high output power as well as high beam quality, are of considerable commercial and academic interest. For example, optical fiber lasers that provide a low M2 (a measure of beam quality) and continuous wave (CW) output powers in the range of tens to hundreds or even thousands of Watts have many industrial applications, as do pulsed lasers having good beam quality and high peak powers. In a fiber lasers having a “master oscillator-power amplifier” (MOPA) architecture, a low power laser source (the “master oscillator”) provides an input “seed” beam to an optical fiber amplifier (the “power amplifier”) that amplifies the seed beam. Unfortunately, nonlinear phenomena, such as Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS), can severely limit scaling the output power of such a fiber laser to higher powers. Furthermore, avoiding nonlinear phenomena as well as maintaining good beam quality can be difficult. Good beam quality typically requires single transverse mode operation of the MOPA, which in turn typically requires optical fibers having cores of relatively small cross sectional area. However, a small cross sectional area results in a higher power density, and high power density more readily triggers nonlinear effects. Increasing the cross sectional are of the core lowers the power density, and helps avoid nonlinearities, but cores having larger cross sectional areas are typically multimode, which tends to degrade the beam quality and hence raise the M2 parameter.
U.S. Pat. No. 5,818,630 teaches one approach for maintaining good beam quality in fibers having cores having larger cross sectional areas. The '630 patent teaches a mode converter that receives an input beam from a laser seed source having a nearly diffraction limited mode. The mode converter converts the mode of the input beam to match a fundamental mode of a multimode fiber amplifier, providing a mode converted input beam to the multimode fiber amplifier. Because the optical energy delivered to the fiber amplifier is matched to the fundamental mode, the amplifier, despite being a multimode amplifier, provides at an output thereof an amplified beam in the fundamental mode. Keeping the multimode fiber of the multimode amplifier as straight as possible helps avoid mode coupling between the fundamental and higher order modes. Mode coupling is undesirable, as energy transferred to higher order modes would then be amplified by the multimode amplifier, which would degrade beam quality. However, the '630 patent indicates that coiling of the fiber can be tolerated without incurring detrimental mode coupling.
It is an object of the present invention to provide improved methods and apparatus for proving higher optical powers from optical fiber devices such as optical fiber lasers and amplifiers.